Your Royce Union should be English threaded so you should be able to do the switch w/o too much hassle. Any older bike that's not a collectible can benefit greatly from this swap.

Jonny: Both types are piece cranks. Modern (been around awhile though) is cotterless. Cottered cranks are usually chromed steel (aluminum has been used but rare) and the cranks are held to the BB spindle by threaded bolts and nuts called cotter pins. They pass through the crank arm where it fits over the spindle and press against the spindle holding the crank arm in place. For a long time they were standard on bikes world wide (US made Schwinn the major exception). Pretty much extinct today except for kid's bikes and some real lowliners.